1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for manufacturing the device, which is formed of a III-V group compound semiconductor, such as AlxGayAs1-x-y (0≦x≦1, 0≦y≦1) or AlxGayInzP1-x-y-z (0≦x≦1, 0≦y≦1, 0≦z≦1) and suitably used for fields of optical recording, optical reproducing, optical communication, optical measurement, etc.
2. Description of the Related Art
In recent years, a vast amount of information has been processed by an information communication machine, and a recording machine/medium having a high speed and a large-capacity is highly demanded. In a DVD-R/RW drive unit, for example, used is a red semiconductor laser with high output and efficiency and an oscillation wavelength of about 650 nm. Faster processing requires a semiconductor laser with higher output and efficiency. Nowadays, an AlGaInP-based semiconductor laser with an optical output of 200 mW or more is under development.
FIG. 9 is a sectional view showing an example of a conventional semiconductor laser device. On a substrate 57 of an n-type GaAs, formed are an n-type buffer layer 56, an n-type cladding layer 55, an active layer 54, a p-type cladding layer 53, a p-type BDR (Band Discontinuity Reduction) layer 52, and a p-type cap layer 51, in this sequence using MOCVD (Metal Organic Chemical Vapor Deposition).
On the upper face of the p-type cap layer 51, a p-side electrode (not shown) is provided, and on the lower face of the substrate 57, an n-side electrode (not shown) is provided.
The n-type buffer layer 56 is formed of a GaAs semiconductor doped with Si (silicon) as an n-type dopant.
The n-type cladding layer 55 is formed of an AlGaInP semiconductor doped with Si as an n-type dopant.
The active layer 54 has a MQW (Multiple Quantum Wells) structure in which barrier layers 54a and well layers 54b are alternately laminated and optical guide layers 54c are formed on both the outer sides. The barrier layers 54a are formed of an AlGaInP semiconductor, the well layers 54b are formed of a GaInP semiconductor, the optical guide layers 54c are formed of an AlGaInP semiconductor, and these layers are non-doped layers without any dopant.
The p-type cladding layer 53 is formed of an AlGaInP semiconductor doped with Zn (zinc) as a p-type dopant.
The p-type BDR layer 52 is formed of a GaInP semiconductor doped with Zn as a p-type dopant.
The p-type cap layer 51 is formed of a GaAs semiconductor doped with Zn as a p-type dopant.
Generally, the p-type BDR layer 52 has a concentration of dopant nearly equal to that of the p-type cladding layer 53. For example, the p-type BDR layer 52 and the p-type cladding layer 53 have the carrier concentration of 1.5×1018 cm−3.
The p-type cap layer 51 generally has a higher concentration of dopant of, e.g., 2.0×1019 cm−3 so as to attain a better ohmic contact with the p-side electrode.
The related prior arts are listed as follows: Japanese Patent Examined Publication (kokoku) JP-B-7-32285 (1996), and Japanese Patent Unexamined Publications (kokai) JP-A-11-87832 (1999), JP-A-2000-68597, and “F. Brunner, et al., Journal of Crystal Growth 221 (2000) pp. 53-58.”
Zn used for the p-type dopant has properties of being easily diffused in a process of crystal growth or heating. Therefore, Zn doped in the p-type cap layer 51 is diffused inside of the p-type BDR layer 52 and also into the p-type cladding layer 53 according to the gradient of the dopant concentration, thereby increasing not only the dopant concentration of the p-type BDR layer 52 but also the dopant concentration of the p-type cladding layer 53. If the Zn-diffusion proceeds thorough the p-type cladding layer 53 to the non-doped active layer 54, the active layer 54 has a nonluminous recombination center inside, thereby degrading the laser characteristics.
In order to suppress this phenomenon, conventionally, by reducing the carrier concentration of each p-type layer having a large resistance, i.e., the p-type cap layer 51, the p-type BDR layer 52 and the p-type cladding layer 53, the Zn-diffusion into the active layer 54 and the degradation of the laser characteristics are suppressed.
However, in case the carrier concentration of each p-type layer is lowered too much, the resistance of the device may be increased and the discontinuity of the conduction band (ΔEc) near the interface of the active layer 54 may be lowered, causing an increase of the operation current at a high temperature.
Meanwhile, Mg acting as the p-type dopant has a diffusion factor smaller than that of Zn, but when doping a GaAs layer with Mg so as to keep a mirror surface without surface irregularities, the maximum doping amount is thought to be nearly equal to 1.5×1019 cm−3. Since this level of the doping amount hardly attain a low contact resistance between the p-type cap layer 51 and the p-side electrode, another device structure, in which Zn is used for the p-type dopant in the p-type cap layer 51 and Mg is used in the other p-type layers (the p-type BDR layer 52 and the p-type cladding layer 53), is also employed.
In this device structure, since the p-type dopant remaining near the active layer 54 is low-diffusive Mg, diffusion of the dopant into the active layer 54 may be inhibited even if a high concentration of the p-type dopant remains near the active layer 54. However, both Zn being doped in the p-type cap layer 51 and Mg being doped in the p-type BDR layer 52 and the p-type cladding layer 53 belong to the same II-group element and have the same mechanism of the dopant diffusing into a crystal. Consequently, such a diffusion phenomenon reportedly occurs that Zn and Mg can mutually diffuse into the crystal at a extremely high speed.
FIG. 4 is a graph showing an example of profiles on concentration of each dopant. As shown in FIG. 4, Zn doped in the p-type cap layer 51 can reach near the active layer 54. A large amount of Zn is likely to concentrate into the active layer 54 as heat cycle is repeated, thereby increasing the nonluminous recombination centers inside the active layer 54 and degrading the laser characteristics.
Another concept of a Zn-free semiconductor laser device structure (excluding Zn) is proposed against such a background, in which a low diffusive dopant, alternative to Zn, is doped in all the p-type layers (the p-type cap layer 51, the p-type BDR layer 52, and the p-type cladding layer 53 in FIG. 9). In case C (carbon) is doped in a GaAs layer, for example, C is thought to has a extremely low diffusion factor and hardly diffuse even after repetition of heat cycle.
In case the p-type cap layer 51 is doped with C and the p-type BDR layer 52 and the p-type cladding layer 53 are doped with Mg, mutual diffusion of the same group element cannot occur because C (carbon) belongs to the IV group element.
However, if the p-type BDR layer 52 has a small doping amount of Mg, the resistance of the device is increased due to reduction of the carrier concentration in the interface between the GaAs layer and the GaInP layer.
As described above, in the combination of the Zn-doped p-type GaAs cap layer and the Zn-doped p-type GaInP BDR layer and the other combination of the Zn-doped p-type GaAs cap layer and the Mg-doped p-type GaInP BDR layer, either Zn in the cap layer may diffuse into the BDR layer or Zn and Mg may mutually diffuse, so that a extremely higher concentration of the p-type dopant tends to distribute in the vicinity of the interface between the GaAs layer and the GaInP layer, as compared to the predeterminate concentration of the dopant.
FIG. 4 shows the combination of the Zn-doped p-type GaAs cap layer and the Mg-doped p-type GaInP BDR layer. The predeterminate concentration of Mg in the BDR layer is 1.5×1018 cm−3, but the actual line shows a concentration higher than 1.5×1018 cm−3 due to Zn-diffusion from the cap layer.
Meanwhile, FIG. 3 shows yet another combination of the C-doped p-type cap layer and the Mg-doped p-type BDR layer. Since C has a low diffusion factor and does not mutually diffuse, diffusion of the p-type dopant hardly occur in the vicinity of the hetero-interface between the cap layer and the BDR layer. Therefore, in case the p-type BDR layer 52 has a small doping amount of the p-type dopant, the carrier concentration on the hetero-interface is reduced, thereby causing increases of the resistance and the differential resistance of the device (see FIG. 5).